Today's world economy is built on the basis of fossil energy, such as coal, petroleum and natural gas. However, these non-renewable fossil energy sources are running out continuously. Since the beginning of the 21st century, global energy problems and the associated problems like environmental pollution and climate warming have become more apparent and severer. Solar energy has the prominent advantages such as wide distribution, abundance, pollution-free, cleanliness, safety, and easy accessibility, and is therefore considered to be one of the most promising renewable energy sources. Solar cells convert sunlight directly to electrical energy, and are practical and effective means to utilize solar energy. However, the commercialized solar cells so far are still limited to silicon based inorganic solar cells, of which the price is so high that it exceeds the generally acceptable level, which greatly limits the range of their applications. In order to reduce the cost of cells and to broaden the range of applications thereof, people have long been looking for novel materials for solar cells.
Organic solar cells are novel solar cells. Comparing with inorganic semiconductor materials which has the disadvantages such as limited sources, high price, toxicity, complicated preparation process, and high cost, organic solar cells possess certain incomparable advantages such as wide range of sources of materials, structural diversity and adaptability, low cost, safety and environmental friendliness, simple production process, light weight and possibility for large area flexible preparation. Therefore, organic solar cells can be widely used in a variety of areas like construction, lighting and power generation, and possess an important development and application prospect. In this connection, many domestic and foreign research institutions and enterprises have paid considerable attention and investment. However, so far, the photoelectric transformation efficiency of organic solar cells is still much lower than that of inorganic solar cells. Therefore, the development of novel organic semiconductor materials has great significance for improving the efficiency of organic solar cells.
Currently, the application of silicon cells for ground use is limited due to its complicated production process and high cost. In order to reduce the cost of the cells and to increase the range of applications thereof, people have long been looking for novel material for solar cells. Organic semiconductor materials have drawn great interest due to their advantages of readily availability of materials, low cost, simple preparation process, good environmental stability and good photovoltaic effect. Since 1992 when N. S. Sariciftci et. al. reported in SCIENCE the phenomena of photo-induced electron transfer between a conjugated polymer and C60, lots of efforts have been made in the research of polymer solar cells, and a rapid development had been obtained. However, the conversion efficiency of organic solar cells is still much lower than that of inorganic solar cells. The constraint factors that limit the improvement of the performance include: mismatch of the spectral response of an organic semiconductor materials with the solar radiation spectrum, and the relatively low migration rate and electrode collection rate of the charge carrier of organic semiconductor materials. In order to practically use polymer solar cells, developing novel materials and significantly improving their energy conversion efficiency are still the primary task in this research area.